1. Technical Field
The present invention relates generally to a data reproduction apparatus, and more particularly to a variable transfer rate data reproduction apparatus having an improved structure designed to read recorded data out of a recording medium which is recorded at a variable transfer rate.
2. Background Art
Various types of high-density recording/reproducing optical discs are known in the art. For example, a compact disc (CD) and a compact disc-interactive (CD-I) are known as a reproduction only optical disc. A magneto-optical disc and a mini disc (MD) are known as a writable optical disc.
The CD-I records therein information signals, which are compressed with ADPCM (Adaptive differential PCM), at a fixed transfer rate determined according to a sampling cycle. The MD records therein compressed information signals at a constant transfer rate.
The amount of data recorded on the CD-I or the MD processed per unit time during reproduction is usually constant. The data processing operation during such reproduction will be described below with reference to FIGS. 7 and 8(a) to 8(e). FIG. 7 shows an example of a conventional structure of a fixed transfer rate data reproduction system. FIGS. 8(a) to 8(e) show an operation of the reproduction system shown in FIG. 7.
The fixed transfer rate data reproduction system, as shown in FIG. 7, includes generally a signal reproduction unit 48, a data storage unit 49, a decoder 52, and a reproduction controller 51. The signal reproduction unit 48 is composed of an optical reproduction element such as an optical head and a signal processing circuit (not shown), and reproduces data recorded on a recording medium 1 such as an optical disc. The decoder 52 outputs a reproduced data signal through an output terminal 53.
An original signal to be recorded on the recording medium 1, as shown in FIG. 8(a), contains data of a constant amount along a time axis, which is, as shown in FIG. 8(b), compressed uniformly (by one-fourth in the shown example) to be recorded on the recording medium 1.
In operation, the optical disc 1 is first rotated at a given speed by a rotary mechanism (not shown). The optical head of the signal reproduction unit 48 then reproduces the recorded data on the optical disc 1 under control of a focusing control system and a tracking control system in the following manner.
The optical head emits a laser beam to a signal surface of the optical disc 1 to form an optical spot thereon. A photosensitive device (not shown) of the optical head receives a reflected beam from the optical disc 1, and converts it into an electric signal. The electric signal is then subjected to a given signal processing and outputted to the data storage unit 49 as reproduced data. The data storage unit 49 then stores therein the reproduced data.
If the compressed data, as shown in FIG. 8(b), recorded on the optical disc 1 is transferred, in sequence, directly from the signal reproduction unit 48 to the decoder 52, the reproduced data four times as much as the original signal per unit time, which is beyond a decoding capacity of the decoder 52, is transferred to the decoder 52.
Accordingly, the amount of the reproduced data within the decoding capacity is transferred to the decoder 52 in the following manner.
The signal reproduction unit 48 outputs the compressed data recorded on the optical disc 1 to the data storage unit 49 as the reproduced data at a maximum readout rate during a reproduction operation (labeled "REPRODUCTION" in FIG. 8(c)).
Alternatively, the signal reproduction unit 48 takes no action during a interval (labeled "STANDBY" in FIG. 8(c)) between the reproduction operations. Specifically, the signal reproduction unit 48 performs the reproduction operation cyclically under control of the reproduction controller 51, thereby allowing the reproduced data of an amount within the decoding capacity of the decoder 52 to be transferred to the decoder 52.
The cyclic reproduction operation of the signal reproduction unit 48 is accomplished in the following manner.
During the reproduction operation, the reproduced data outputted from the signal reproduction unit 48 is stored in the data storage unit 49. Conversely, during the standby operation, the reproduced data from the signal reproduction unit 48 is held from being stored in the data storage unit 49. At the same time, a track Jump is generated over one track every rotation of the optical disc 1, so that the optical head of the signal reproduction unit 48 traces the same track repeatedly.
The amount of data, as shown in FIG. 8(c), derived by reproducing the compressed data during the reproduction operation becomes equal to that of the reproduced data, as shown in FIG. 8(d), to be stored in the data storage unit 49.
The data storage unit 49 supplies to the decoder 52 the reproduced data of an amount, as shown in FIG. 8(d), required for the decoder 52 to provide reproduction signals, in sequence, to the output terminal 53.
As appreciated from the above, the fixed transfer rate data reproduction system shown in FIG. 7 reproduces data at a constant transfer rate, so that the amount of the reproduced data per unit time outputted from the signal reproduction unit 48 to the data storage unit 49 is equal to that of data per unit time which is supplied from the data storage unit 49 to the decoder 52. The amount of data is not changed partially during the reproduction operation so that the amount of the reproduced data per unit time transferred from the data storage unit 49 to the decoder 52 is maintained constant. Thus, the transfer of the data of an amount within the capacity of the decoder 52 reproduced by the signal reproduction unit 48 to the decoder 52 may easily be accomplished only by performing the reproduction operation and the standby operation alternately under the control of the reproduction controller 51.
In the above discussed fixed transfer rate data reproduction system, the amount of reproduced data per unit time is, as mentioned above, always constant. This means that the recorded data on the recording medium 1 is reproduced at a constant transfer rate regardless of the contents of information contained in the original signal. It is thus difficult to record and reproduce the original signal for a long period of time.
In order to overcome such a drawback, there has been proposed a high-efficiency coding system in accordance with variable transfer rate coding (i.e., a variable transfer rate coding system) for more efficient use of a limited recording capacity of a recording medium. In this coding system, a small number of codes per unit time are assigned to part of contents of information contained in the original signal which does not impinge upon decoding, while a great number of codes are assigned to part of the contents of information which impinges upon the decoding.
When recording data which is coded according to the above variable transfer rate coding is recorded on a recording medium at a preselected recording density, it is possible to record the recording data at twice a recording density at which the recording data is coded in accordance with the fixed transfer rate coding, however, the amount of reproduced data per unit time decoded in a decoder becomes equal to that of the coded data, as shown in FIG. 5(a), which is changed extremely.
The variable transfer rate coding system, similar to the data reproduction system shown in FIGS. 7, and 8, has the signal reproduction unit 48 and the data storage unit 49. The signal reproduction unit 48 performs a reproduction operation cyclically. The data storage unit 49 stores data reproduced by the signal reproduction unit 48 during the cyclic reproduction operation, and transfers a given amount of the reproduced data to the decoder 52 at regular intervals. When the amount of the reproduced data transferred to the decoder 52 is changed considerably, the decoder 52 may not decode consecutive original signals.
In order to avoid such a drawback, it may be proposed to constantly transfer a sufficient amount of reproduced data from the data storage unit 49 to the decoder 52 under control of a CPU incorporated in the reproduction controller 51. It is however difficult for the CPU to perform such control due to a lack of signal processing capacity.
In order to alleviate such a problem, it may also be useful to employ additional hardware and a CPU which is capable of controlling the data storage unit 49 so as to constantly supply a sufficient amount of reproduced data to the decoder 52 while performing normal reproduction control. These arrangements however increase the entire size of the system as well as its manufacturing costs.
The fixed transfer rate data reproduction system, as discussed above, generates a track jump over one track turn every rotation of the optical disc 1 during the standby operation. However, if a ratio of the amount of data read out of the optical disc 1 through the signal reproduction unit 48 to the amount of data processed by the decoder 52 is high, the length of time during which the standby operation is performed becomes greater than that during which the reproduction operation is performed, thus resulting in increased track jumps over one track turn, which will be described below with reference to FIG. 6(a).
The shown optical disc 1 has a spiral track. A thick line extending from a to b indicates a portion of the track from which recorded data is read out during the reproduction operation. A broken line extending from b to c indicates a portion of the track which an optical spot formed by the optical head traces during a subsequent standby operation.
During this standby operation, the optical spot traces the portion of the track indicated by the broken line from b to c (i.e., subsequent one track turn) repeatedly with track jumps or alternatively jumps from b to a to trace the portion indicated by the thick line repeatedly.
The track jump of the optical spot is achieved by supplying a kickback pulse to an actuator incorporated in a tracking control system of the signal reproduction unit 48.
Therefore, as discussed previously, when a compression ratio of the recorded data to the original signal is high and a ratio of the length of time of the standby operation to that of the reproduction operation is also high, the number of the kickback operations (i.e., the track jumps of the optical spot) of the actuator is increased. This results in increased mechanical stress acting-on a support structure and a drive coil of the actuator, thereby leading to a decreased lifetime of the system. Additionally, in the worst case, a circuitry around the tracking control system is broken, resulting in a malfunction of the actuator. Further, if the kickback pulse is applied to the actuator to move the optical head over a defect of the optical disc 1, a large track jump may occur.
When the standby operation is switched to the reproduction operation, it is necessary for the optical spot to be shifted toward a track turn which is to be reproduced subsequently. When a track Jump is generated during this period, the optical head needs to move over a plurality of track turns. This may require a long period of time for resuming the reproduction operation. In such a case, the need is arisen for providing a large storage capacity of a memory in the data storage unit 49.
Additionally, when the time required for resuming the reproduction operation is much longer than estimated, a sequence of reproduced data transferred from the data storage unit 49 to the decoder 52 may be broken, so that a reproduced signal is not outputted.
When the readout of reproduced data from the signal reproduction unit 48 is effected cyclically through the reproduction operation and the standby operation in the manner described above, it is effective to use a dynamic random-access memory (D-RAM) as a buffer memory in the data storage unit 49 for supplying a sufficient amount of data to the decoder 52.
As is well known in the art, it is commonly essential to refresh a D-RAM at given intervals by rewriting its entire contents. Thus, such a refreshing operation is needed in addition to the reproduction operation and the standby operation.
Thus, if the writing operation, the readout operation, the refreshing operation each require a period of time T. as shown in FIG. 11, three time periods 3T complete one memory cycle.
FIG. 12 shows circuit arrangements of a data storage unit 74 identical with the data storage unit 49.
The data storage unit 74 has disposed therein a D-RAM 61 which stores reproduced data inputted to an input terminal 75.
The reproduced data read out of the D-RAM 61 is decoded by a decoder 26 to provide a reproduction signal through an output terminal 78.
The reproduced data inputted to the input terminal 75 is data which has been read out of the optical disc 1 and subjected to a given signal processing through the signal reproduction unit 48.
The data storage unit 74 includes the D-RAM 61, a timing signal generator 79, a write address inhibiting circuit 80, a write address counter 81, a refreshing address counter 82, a readout address inhibiting circuit 83, a readout address counter 84, an address strobe pulse generator 85, a write enable signal generator 86, and an address multiplexer 70.
As long as the amount of reproduced data stored in the D-RAM 61 is held within a given storage range, the data storage unit 74 supplies the reproduced data to the decoder 26 and stores or writes therein the reproduced data inputted to the input terminal 75.
To an terminal 76, a clock signal is inputted from the signal reproduction unit 74. To a terminal 77, positional data (e.g., subcode data) of recorded data on the optical disc 1 which is to be reproduced therefrom is inputted from the signal reproduction unit 74.
When data reproduced from the optical disc 1 is written into and out of the D-RAM 61 and the D-RAM 61 is refreshed at the cycles shown in FIG. 11, address signals provided, respectively, by the write address counter 81, the readout address counter 84, and the refreshing address counter 82 are inputted to the D-RAM 61 through the address multiplexer 70 in accordance with time-division multiplexing. This allows the write operation, the readout operation, and the refresh operation of the D-RAM 61 to be performed.
The high-speed signal processing of data reproduced from the optical disc 1 in the signal reproduction unit 48 requires use of a high-speed D-RAM as the D-RAM 61. However, a desired high-speed D-RAM may be unavailable for a lack of operational speed or economical reasons.
Further, the data reproduction system designed to reproduce data formed by recording high-efficiency coded image data through a variable transfer rate system encounters the following drawbacks.
For example, reproduced data from the optical disc 1 may be stored once in the buffer memory of the data storage unit 49 at a first transfer rate and then outputted to the decoder 52 at a second transfer rate. Such techniques are taught, for example, in Japanese Patent First Publication No. 4-186563 as a digital data reproduction system.
This conventional digital data reproduction system is designed to set a rotational speed of a disc to a value suitable for a data transfer rate of an external device such as a computer for outputting data at a transfer rate required by audio or image data, and uses a random access memory as a buffer memory.
Moreover, Japanese Patent First Publication No. 4-181547 discloses a data reproduction system for a disc unit having disposed therein a storage device similar to the above mentioned buffer memory. This Publication also discloses a data reproduction system which brings a reproduction operation of a disc into an idling mode for a given period of time when a constant amount of data is stored in a storage device. The time required to compensate for a track shift is shortened by repeating the idling mode.
FIG. 18 shows an example of a conventional data reproduction system having a memory device like the one described above.
A recording medium or optical disc 100 is rotated by a spindle motor 120. Data recorded on the optical disc 100 is read out by an optical pickup (labeled "PU" in the drawing) and transferred as an analog reproduction signal S10 to a wave shaper 160.
The analog reproduction signal S10 is wave-shaped and binary-coded by the wave shaper 160 to provide binary-coded data S12 to a channel decoder 180 and a PLL circuit 200, respectively.
The PLL circuit 200 generates latch pulses S14 using the binary-coded data S12 from the wave shaper 160 to output them to the channel decoder 180 and a servo circuit 260.
The channel decoder 180 is responsive to the latch pulse S14 from the PLL circuit 200 to latch the binary-coded data S12 inputted from the wave shaper 160.
Subsequently, the channel decoder 180 detects synchronization of the latched binary-coded data S12, detects an error under CIRC (centralized Information Reference and Control), and corrects the error to provide data S16 to a buffer RAM 220.
The channel decoder 180 outputs subcode data S18 containing sector address extracted from the binary-coded data S12 to an access control unit 240.
The servo circuit 260 is responsive to the latch pulse S14 supplied from the PLL circuit 200 to control the speed of the spindle motor 120 to agree with a reference clock generated by a crystal oscillator (not shown) disposed in the channel decoder 180.
The servo circuit 260 is also responsive to an access control. signal S20 supplied from the access control unit 240 to operate tracking actuators (not shown) built in the optical pickup 140 and a sliding motor 280. With these arrangements, the optical pickup 140 is moved to a desired sector on the disc 100.
The buffer RAM 220 stores therein the data S16 from the channel decoder 180 according to a reproduction timing signal (not shown) outputted from the channel decoder 180. The data S16 stored in the buffer RAM 220 is read out according to a request signal S22 provided from an external signal processing unit 300 to a R/W cycle control unit 320, and then supplied to the external signal processing unit 300 as a readout data S24.
Writing and reading of data into and out of the buffer RAM 220 in the data reproduction system shown in FIG. 18 will be described below with reference to FIG. 19 which shows a data-storing process of the buffer RAM 220.
The R/W cycle control unit 320 monitors the amount of data stored in the buffer RAM 220 to control reading and writing of data out of and into the buffer RAM 220.
An address comparator 340 determines whether or not the amount of data stored in the buffer RAM 220 exceeds a minimum level (i.e., lower limit) shown in FIG. 19 or not. Note that the minimum level is fixed.
If the address comparator 340 has concluded that the amount of data stored in the buffer RAM 220 is decreased below the minimum level, it provides a readout inhibit signal S26 to the external signal processing unit 300, so that the request signal S22 is inhibited from being outputted.
An address comparator 360 determines whether or not the amount of data stored in the buffer RAM 220 exceeds a maximum level (i.e., upper limit) shown in FIG. 19. Note that the maximum level is fixed.
If the address comparator 360 has concluded that the amount of data stored in the buffer RAM 220 is increased above the maximum level, it provides a reproduction inhibit signal S28 to the access control unit 240 to store a sector address on the disc 100 during reproduction and to inhibit data on a subsequent sector from being reproduced.
Afterwards, the external signal processing unit 300 reads the data stored in the buffer RAM 220. When the amount of data stored in the buffer RAM 200 is decreased below the maximum level, the subsequent sector following the sector on which the data is stored and reproduced previously is accessed for resuming the reproduction of data.
When the amount of data stored is increased over the maximum level again, the reproduction of data is stopped, while when it is decreased below the maximum level again, the reproduction of data is resumed. In this manner, the data reproduction operation is, as can be seen from FIG. 19, performed cyclically.
Note that during a period of time until the amount of data stored in the buffer RAM 220 reaches the minimum level following initiation of a data storage operation, the data reproduction system continues to store data in the buffer RAM 220, while the external signal processing unit 300 is inhibited from reading the data out of the buffer RAM 220. Thus, the data is stored in the buffer RAM 220 at the same rate as a maximum reproduction transfer rate of the optical disc 100.
During a period of time until the amount of data stored in the buffer RAM 220 reaches the maximum level over the minimum level, both the writing data into and the reading data from the buffer RAM 220 are allowed. The data is stored in the buffer RAM 220 at a rate of a data writing rate minus a data reading rate.
Subsequently, when the amount of data stored in the buffer RAM 220 exceeds the maximum level, the data reproduction system is brought into a kickback state on standby, inhibiting the data from being stored in the buffer RAM 220.
When the amount of data stored in the buffer RAM 220 is decreased below the maximum level again, the optical pickup is shifted to search a subsequent sector on the disc 100 following a sector from which data is last read out.
In FIG. 19, search periods are indicated by broken lines. Upon access to a desired sector on the disc 100, data is read out therefrom and then written into the buffer RAM 220.
Once the amount of data stored in the buffer RAM 220 exceeds the maximum level after initiation of the data storage operation, it varies above and below the maximum level unless the data reproduction system is required to perform an access operation in response to a request signal inputted externally or a servo-malfunction occurs.
Specifically, the writing of data into the buffer RAM 220 is effected intermittently, so that three processes: the reading of data from one sector, the standby in the kickback operation, and the track search by movement of the optical pickup are performed in sequence. In other words, mechanical operations such as a track search and a track jump are required frequently, thereby causing mechanical parts to be degraded prematurely.